Electrographic record medium

ABSTRACT

For use in an electrostatic recording system with voltage charging apparatus having charging electrodes, an electrographic record medium is provided having spacer means, a portion of which projects above the outer surface of a dielectric layer of the record medium. The spacer means space the outer surface of the dielectric layer from the charging electrodes during the voltage charging of a defined area of the outer surface of the dielectric layer.

United States Patent Brown, Jr. et al.

3,657,005 Apr. 18, 1972 ELECTROGRAPHIC RECORD MEDIUM Adi; P12?BS9191Jfisc k filili jfl bl i John Blumenthal, Wickliffe, both of Ohio Clevite Corporation Dec. 29, 1967 Inventors:

Assignee:

Filed:

App]. No.:

U.S.Cl ..ll7/20l, 117/215, 117/217, 117/155, 117/175 Int. Cl. ..H0lf 11/02, H011 10/06 FieldofSearch ..l17/20l,215,2l7,216, 155 L, ll7/l7.5

References Cited UNITED STATES PATENTS 3,097,964 7/1963 Stowell ..117/155 Primary Examiner-William L. Jarvis Attorney-Eber J. Hyde [57] ABSTRACT For use in an electrostatic recording system with voltage charging apparatus having charging electrodes, an electrographic record medium is provided having spacer means, a portion of which projects above the outer surface of a dielectric layer of the record medium. The spacer means space the outer surface of the dielectric layer from the charging electrodes during the voltage charging of a defined area of the outer surface of the dielectric layer.

7 Claims, 7 Drawing Figures PULSE VOLTAGE CHARGING APPARATUS ELECTROGRAPHIC RECORD MEDIUM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an electrographic record medium. More particularly, this invention relates to the charge retentive surface of the electrographic record medium for use in a system of high speed electrostatic recording with voltage charging apparatus and wherein electrostatic latent images are impressed upon the charge retentive surface of the record medium.

In electrostatic recording the indicia to be printed, such as letters, lines, dots, etc., are first formed as invisibleelectrostatically charged surface areas of an electrographic record medium by means of electrical discharges from suitably shaped and suitably positioned electrodes. The desired size and shape of the charged surface area of the record medium is the size and shape corresponding to an exact reproduction of the charging electrode. The present invention is particularly applicable to an electrographic system using pulse voltage charging to provide a charged area on the surface of the record medium. Subsequently, these charged areas are rendered visible by the application of a toner to the surface of the record medium as a developing agent or ink. The toner particles are held to the charged areas of the record medium by electrostatic attraction.

Voltage charging is herein defined as the electrical charging of an area (as defined by the charging electrode of the dielectric surface of the record medium by momentarily raising the electrical potential of the charging electrode with respect to a conductive stratum of the record medium during a period of relatively no displacement between the electrode and the surface to be charged. Thus, the electrical charging potential may be of any time duration as long as there is essentially no relative displacement between the charging electrode and the record medium. This is in contrast to d-c charging where the electrical potential remains on the charging member during a period of relative displacement (perpendicular or lateral) between the latter and the record medium. This definition is to distinguish the most useful application of the present invention, i.e., pulse voltage charging type of electrostatic recording, from d-c writing the record member of this invention is applicable to both systems.

2. Description of the Prior Art In establishing electrically charged areas on a record medium with pulse voltage charging, it is well known in the art that a gap or space must exist between the charging electrode and the surface of the record medium to be charged. If the charging electrode is in intimate contact with the surface, and no displacement between the two occurs in the period during which the pulse voltage is applied, the potential on the dielectric surface of the record medium will follow the potential on the charging member. That is, the area actually contacted by the charging electrode will return to zero potential along with the electrode resulting in no remanent electrical charge on the record medium.

In practice it is extremely difficult to provide true electrical contact between the contacting surface area of the charging electrode and the corresponding area on the dielectric surface. The existence of some minute gap, even though infinitesimal, may generally be assumed for most of the area involved. For this reason, the charging of the surface area involves transporting a net electrical charge across a gaseous medium, generally air at atmospheric pressure and at room temperature for practical charging arrangements. The nonlinear electrical conduction in gases causes the charges transported to the surface of the dielectric layer of the record medium to become trapped thereon, rather than being drained off again through the charging electrode, as would be the case if true contact to the dielectric layer had beenestablished.

As a result of knowing a gap must exist, many different approaches have been taken in the prior art to create the gap, and different opinions and theories have been expressed as to length of the gap. In all cases, the magnitude of the gap is extremely small, on the order of thousandths of an inch and less.

At this point it is felt to be necessary to discuss the effect of varying the gap length on the voltage required to charge a defined surface area. This will show the critical nature of having the proper gap length and thereby demonstrate the necessity for providing proper gap spacing. This can best be seen by a general description of the results observed from experimentation since the theoretical explanations of the prior art appear to be contradictory. When operating at atmospheric pressure and with extremely small gap distances, less than 0.2 mil, the voltage required to produce charging of the surface of the dielectric layer increases rapidly as the gap length is decreased. The required voltage increases so rapidly that for gap lengths of about 0.04 mil and less, the charging becomes virtually impossible for any practical system. If the gap length is increased from about 0.25 mil, the voltage required for charging also increases but at a more gradual rate than when the gap length is decreased below 0.2 mil. Increasing the gap length above about 0.25 mil does not result in exorbitantly high voltages but does result in substantially higher required voltages and in harmful side effects, mainly, spreading and loss of resolution of the electrostatic latent image on the surface of the record medium.

The prior art teaches that a gap length in the order of 0.1 mil to 0.25 mil or larger is preferable. Generally speaking, the prior art has tried to create a gap length in this range by having the record medium supported in a parallel planar relationship with the charging electrodes. In most cases, the record medium rests on the inner surface of a lower supporting means. The interior of the upper means, which carries the charging electrodes, is spaced out of contact with the record medium. Such a method is severely limited by incremental thickness variations in the vicinity of the charging electrode; and also may be severely limited by gross sheet thickness variations across the entire width of the record medium. In addition, it is difficult and expensive to make a charging head several inches in length with a uniform tolerance within about 1 mil. It should be remembered that the magnitudes when discussing the gap length are extremely small and that to hold paper tolerances to such limits would be very expensive. In order to achieve the precise gap spacing taught by the present invention, the prior art paper and/or equipment tolerances would have to be in the order of tenths of a mil.

Another major factor is the time duration of the pulse applied to the charging electrode. Increasing the magnitude of the applied voltage will improve reliability. However, care must be taken not to transfer excessive charge to the charging electrode since the charge spreads laterally at the record surface. On development, this spreading of charge manifests itself in image deformities.

The present invention provides a record medium which is reliably charged by conventional charging apparatus with pulses as low in voltage as about 450 V. and durations at least as short as 0.5 microsecond.

It should be noted, however, that the discharge is not solely influenced by the gap length and pulse time duration but also by several other conditions, including humidity and pressure of the ambient atmosphere.

An object of the present invention is to provide, in an electrostatic recording system with pulse voltage charging, a novel solution to the gap spacing problem in that the charging electrodes are spaced from the surface of the record medium to be charged by the record medium itself. Thus, the desired spacing is obtained without external components.

A further object of the present invention is to provide an electrographic record medium which accurately spaces the dielectric surface of the record medium from the charging electrodes to permit the use of the lowest voltage consistent with reliable charging.

A further object of the present invention is to provide an electrographic record medium which provides gap spacing which is readily and economically produced and, in addition, is adaptable to present electrostatic recording systems.

A further object of the present invention is to provide an electrographic record medium which eliminates the need for any external adjustment of the charging electrodes or the record medium to obtain the desired gap spacing.

Another object of the present invention is to provide an electrographic record medium that when in contact with the charging head has at least 80 percent or greater of the dielectric surface at or nearly at the desired spacing.

A further object of the present invention is to provide an electrographic record medium which in itself provides spacing from the charging head while not substantially altering the printed mark.

SUMMARY OF THE INVENTION Briefly, in accordance with the present invention, for use in electrostatic recording with pulse voltage charging apparatus, the electrographic record medium, having a dielectric layer with an outer surface capable of retaining an electrostatic charge, is provided with spacer means projecting above the surface of the dielectric layer for establishing a given space between this surface and the charging apparatus.

The invention will be better understood from the following description of a preferred embodiment to be read in conjunc tion with the accompanying drawing, and the features believed to be novel will be more particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF DRAWING In the drawing:

FIG. 1 is a side elevation of an electrostatic charging head in contact with the electrographic record medium of the present invention;

FIG. 2 is a cross-sectional view along line 2-2 of the electrostatic charging head of FIG. 1 showing the linear array of charging electrodes;

FIG. 3 is an enlarged and sectioned side view of FIG. 1 showing the relationship of one charging electrode to the record medium of the present invention;

FIGS. 4 through 7 illustrate five species of spacer means in the present invention;

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of the drawing, a conventional pulse voltage charging system having a charging head 12 is shown in contact with a moving electrographic record medium 14 which is transported by means not shown. The charging head 12 comprises an array of fine charging electrodes or styli 16 (see FIG. 2). The charging electrodes 16 are generally small, fragile electrical conductors; by way of illustration, the charging electrodes 16 of this present embodiment are approximately 8 mils in diameter and spaced on mil centers. Therefore, they are generally embedded, as in this embodiment, in support 18 preferably composed of a suitable insulating material such as a plastic or ceramic insulator. If stronger electrodes 16 are used, no support is necessary.

In the present embodiment, the charging electrodes 16 are arranged in a linear array (see FIG. 2) for line-type printing; however, the present invention would work equally as well with any arrangement of the charging electrodes 16, for example, a matrix display.

As shown in FIGS. 2 and 3, the charging electrodes 16 are flush with the insulating support 18 at their lower exposed ends 20. Lower exposed ends 20 of the charging electrodes 16 define the electrostatic latent image that results on the record medium and can be of any shape such as letters, lines, dots, etc., to produce a similar latent image on the electrographic record medium 14. The shape of the lower exposed ends 20 of the present embodiment is, for example, a circular area or dot.

The upper ends 22 of the charging electrodes 16, as shown in FIG. 1, are connected to a pulse voltage charging apparatus 24. Apparatus 24 receives electrical signals from a computer or any other high speed electronic pulse creating device and transfers the signals to the proper electrodes.

In the present embodiment, the record medium 14 comprises a conductive base layer 26, a dielectric top layer 28 and spacer means 30 (see FIG. 3). The conductive base layer 26 is preferably a conventional conductive paper, such as those obtained by coating and/0r impregnating with various ionic conductors as used in the art, conductive carbon filled paper or carbon filled coatings on paper. The dielectric layer 28 is preferably a conventional dielectric lacquer coating as used in the art, for example, a polyvinyl acetate, polystyrene, polyvinyl butyral, or polymethyl methacrylate. It is desirable for keeping the applied voltage at a minimum that the dielectric layer be kept as thin as possible in range of 0.1 to 0.3 mils.

In accordance with the present invention and as shown in FIG. 3, a spacing or gap, of length d, between the lower exposed ends 20 of the charging electrodes 16 and the record medium 14 is provided by the medium, and more particularly by spacer means 30. The spacer means 30 can be granular particles of a size that will result in projections above the surface of the dielectric layer to provide the desired spacing, length d. Examples of such spacer means 30 which can be dispersed in the dielectric lacquer are preferably cornstarch, glass shot, refractory particles, or any other particles which when dispersed in or on the dielectric layer 28 provide the required spacing. In an alternative method, the spacing may be provided by altering the surface of the dielectric layer itself to provide the spacer means or by printing the spacer means on the surface. Whether the spacer means may be conductive depends on their relationship to the conductive layer. This aspect of the invention will be explained when discussing the various species of the present invention.

In the charging step of the electrostatic recording process (see FIG. 1), a pulse of electrical energy from the pulse charging voltage supply 24 is applied to the upper ends 22 of charging electrodes 16. This causes a pulse to be applied to the lower ends 20 of charging electrodes 16; thus, a potential is created between the electrodes 16 and the conductive layer 26 which causes a potential across the dielectric layer 28 and gap, of length d. A discharge occurs leaving the surface 32 of the dielectric layer 28 with a charged area 34 preferably corresponding to an exact reproduction in size and shape of the lower exposed end 20 of an electrode 16. During the time the discharge occurs, there is substantially no relative displacement of the charging electrode and the record medium; hence, the defined pulse voltage charging of the surface of the dielectric layer occurs.

As previously mentioned and in accordance with the present invention, the gap, of length d, is created by the projection of the spacer means 30 above the surface 32 of dielectric layer 28. In accomplishing this result by granular particles, the spacer means 30 may be located in three different relationships to the dielectric layer 28 as shown in FIGS. 4 through 6. In addition, the spacer means 30 may also be provided by uniform alteration of the texture of the surface 32 of the dielectric layer 28 by embossing or printing techniques as shown in FIG. 7. In all of these embodiments the essential requirement is that the spacer means 30 project above the surface 32 of dielectric layer 28 from 0.05 mil to 0.4 mil, and preferably 0.2 to 0.25 mil, to result in a gap, of length d.

In the embodiment in FIG. 4, the spacer means 30 reside on the surface 32 of the dielectric layer 28 in which case the particle size of the spacer means 30 would be in the range of about 0.05 to 0.4 mil to provide the requisite gap, of length d. In this embodiment the spacer means 30 can either be nonconductive or conductive particles. In a method of making this embodiment, the particles could be coated onto the dielectric layer out of a dispersion in a liquid which renders the surface tacky so that the particles become adhered. The particles could also be secured thereto by fusing with heat when either the dielectric layer or the particles or both are heat fusible. Suitable particulate or granular materials for this purpose are glass shot, polyethylene, crude hard wax emulsion, or a metal powder, for example, aluminum powder or zinc dust.

In the embodiment in FIG. 5, the spacer means 30 are fixed to the dielectric layer 28 but do not contact the conductive layer 26. In this case, the particle size of the spacer means 30 would be dependent on the depth to which they are situated in the dielectric layer 28. However, the dielectric layer 28 itself is limited in practical thickness 0.1 to 0.4 mil. However, the projection of the spacer means above the surface 32 of dielectric 28 is still in the range of about 0.05 to 0.4 mil. A method of making this embodiment of the present invention is to disperse conductive particles in a second dielectric layer 29. This second dielectric material 29 is then spread over a conductive layer 26 which already has a thin dielectric layer 28 to insulate the conductive particles of the top dielectric layer 29 from the conductive base layer. In this case, the spacer means 30 can also be either conductive or non-conductive since they are not in contact with the conductive layer 26.

In the embodiment shown in FIG. 6, the spacer means 30 are particles embedded in the dielectric layer 28 and are in contact with the conductive layer 26; hence, in this case, the spacer means 30 must be a dielectric material to prevent a direct electrical path to the conductive layer 26. The particle size of the spacer means 30 would be dependent on the thickness of the dielectric layer; however, the projection of the spacer means 30 above the surface 32 is still from 0.05 to 0.40 mil. This is the preferred embodiment since the spacer means 30 can be mixed with the dielectric coating lacquer and deposited on the conductive layer 26. The method of making an electrograph record medium in such a manner is fully discussed later in this specification.

As shown in FIG. 7, alternate species of the spacer means 30 of this present invention can be provided by altering the surface 32 of the dielectric layer 28 or by printing the spacer 7 means on the surface. In altering the surface as shown in spacer means 30a, the dielectric layer 28 can be raised by an embossing process to form uniform ridges which project from 0.05 to 0.40 mil above the surface 32 of the dielectric layer 28. As shown in spacer means 30b, the gap, of length d, can be provided by printing the means 30b on the surface 32 of the dielectric 28. This can be done by conventional printing techniques such as gravure or intaglio printing. In all cases, the printed marks project from 0.05 to 0.4 mil and preferably 0.2 to 0.25 mil above the surface 32 of the dielectric layer 28.

An expedient method of creating a controlled texture is to disperse 0.4 mil diameter spherical particles in the plastic coating lacquer used in preparing the paper. Typically, the dry thickness of the plastic coating is around 0.2 mil. As lacquers tend to flow away from sharp edges or points, the peaks of the embedded spacer means 30 remain essentially free of dielectric material 28, thus forming 0.2 mil projections above the surface 32 of the dielectric 28 material. In an actual laboratory made coating, cornstarch was used as the spacer means 30. The natural size distribution fell between 0.25 mi] and 0.5 mil with a preponderance measuring about 0.4 mil. The dielectric coating had the following components:

100g. polyvinyl acetate 20g. oil soluble phenolic resin 150g. rutile TiO 400ml. methyl ethyl ketone 2.0g. cornstarch In this formula the coloring pigment (TiO is used to impart whiteness to naturally black carbon filled paper.

The dielectric coating so prepared was applied to a conventional carbon filled paper by a No. 15 Myer rod and resulted in dry film having a thickness of 0.2 mil with starch particles projecting about 0.2 mil above the surface and having a distribution of about 4 particles per 100 sq. mils of area. This is approximately 96 percent or better of free surface spaced a controlled distance from the charging electrodes.

The relative ease with which this electrographic record medium was made indicates that volume production would be economical, and that standard coating techniques can be employed in its manufacture. The low use level of spacing material should make it an insi ificant cost factor.

It will be understoo by those skilled in the art that the foregoing and other changes in the form and details may be made therein without departing from the spirit and scope of the invention. In view of the many species suggested, there are an unending number of ways to provide the spacing means 30 of proper size and distribution. Also, if the other factors (other than gap length) which are variables to be considered in surface charging are drastically changed from the generally normal conditions stated herein, there would be a corresponding change in the requirements on the spacer means 30.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electrographic record medium, for use in electrostatic recording with voltage charging apparatus having charging electrode means, comprising:

a dielectric layer having an outer surface for receiving and retaining an electrostatic charge;

and spacer means on said outer surface of said dielectric layer, said spacer means projecting from 0.05 to 0.4 mil above said outer surface of said dielectric layer for establishing a gap of controlled length between the said outer surface of said dielectric layer and the said charging electrode means during a recording operation.

2. An electrographic record medium as in claim I, wherein the spacer means are alterations of said surface of said dielectric and which project from 0.05 to 0.4 mil above said outer surface of said dielectric layer.

3. An electrographic record medium as in claim 1, wherein said spacer means are selected from the group of granular particles consisting of starch, glass shot, refractory particles, and plastic particles.

4. An electrographic record medium as in claim 3, wherein said spacer means are fixed to said dielectric layer.

5. An electrographic record medium as in claim 3, wherein said spacer means are embedded in said dielectric layer.

6. An electrographic record medium as in claim 1, wherein at least percent of said outer surface of said dielectric is established at a controlled space.

7. An electrographic record medium as in claim 1, wherein said spacer means having a distribution on said medium of l to 10 spacer means per l00 sq. mil of surface area of said medium and spacing said head from said surface of said dielectric during the pulse voltage charging of said surface. 

2. An electrographic record medium as in claim 1, wherein the spacer means are alterations of said surface of said dielectric and which project from 0.05 to 0.4 mil above said outer surface of said dielectric layer.
 3. An electrographic record medium as in claim 1, wherein said spacer means are selected from the group of granular particles consisting of starch, glass shot, refractory particles, and plastic particles.
 4. An electrographic record medium as in claim 3, wherein said spacer means are fixed to said dielectric layer.
 5. An electrographic record medium as in claim 3, wherein said spacer means are embedded in said dielectric layer.
 6. An electrographic record medium as in claim 1, wherein at least 80 percent of said outer surface of said dielectric is established at a controlled space.
 7. An electrographic record medium as in claim 1, wherein said spacer means having a distribution on said medium of 1 to 10 spacer means per 100 sq. mil of surface area of said medium and spacing said head from said surface of said dielectric during the pulse voltage charging of said surface. 